Tall oil is an important by-product of the Kraft process for pulping wood, especially wood derived from pine trees. It is usually seen as a resinous, oily liquid and comprises a mixture of rosin acids and fatty acids and may be used in soaps, emulsions, lubricants, fuels and in other applications.
One important, less refined form of tall oil is referred to as CTO (crude tall oil). CTO contains rosins, unsaponifiable sterols, resin acids (such as abietic acid), fatty adds (such as palmitic, oleic and linoleic adds), fatty alcohols, other sterols, and other alkyl hydrocarbon derivatives.
The fatty add fraction of tall oil is referred to as TOFA (tall oil fatty adds) and is of significant interest in many applications such as in production of soaps and lubricants. Many TOFA chemical derivatives (such as esters) are useful in important applications as well. Other components of CTO, such as tall oil rosin, certain sterols, and so forth, are valuable for other uses.
Crude tall oil is obtained by separating black liquor soap from black liquor obtained from the Kraft process. A black liquor soap (or tall oil soap) is separated from the black liquor, and further processed by acidulation. Acidulation of tall oil soap is treatment with a mineral acid, for example sulfuric acid, to yield a CTO stream and a “spent acid” stream. Washing this black liquor soap prior to acidulation is thought to allow more efficient separation of CTO after acidulation; alkaline tall oil plant brine has been employed as the washing medium. This alkaline tall oil plant brine is produced by adjusting the pH of the spent acid.
However, such efforts to wash black liquor soap to improve CTO separation have been complicated by the presence of too much lignin (or lignates depending on the pH)
The presence of too much lignin in the washed soap was believed to cause problems later in separating out CTO after acidulation, and the presence of lignin is undesirable in tall oil. Thus, efforts were made to remove lignin from the soap during washing. Lignin has sometimes been used as a fuel source.
Additionally, calcium sulfate builds up in systems using soap washing. This build-up is likely due to unrecognized calcium in alkaline tall oil plant brine used as wash medium. We have found that fortified brine (sometimes called extract brine or wash filtrate) resulting from washing tall oil soap, carries calcium species with it. When the fortified brine is sent to a weak liquor system (as is commonly done), concentration of the weak liquor would cause calcium to build up there, and return to the black liquor soap feed system. This in turn increases calcium sulfate formed during acidulation.
Calcium sulfate build-up is very undesirable; if left unchecked, calcium sulfate plugs process equipment. In systems using gravity settling or decanting downstream from acidulation, calcium sulfate accumulation makes it necessary to shut decanters down frequently for cleaning. In continuous processing, where centrifuges separate out lignin and inorganic solids, large amounts of calcium sulfate also require shutdown to clean the system. Therefore, calcium sulfate issues appear to have caused soap washing to be abandoned by most, if not all, applicable industry. In addition, the lignin and insoluble inorganics entrain crude tall oil during separation, reducing the efficiency of such systems.
U.S. Pat. No. 3,575,952 describes a process to wash crude tall oil soap with alkaline tall oil plant brine for the purpose of removing lignin. There is no teaching regarding calcium deposits. This patent teaches that the brine should be at a pH of at least about 8 and preferably 10-14.
U.S. Pat. No. 4,248,769 describes a process to wash crude tall oil soap with an alkaline tall oil plant brine in order to remove lignin, where the wash mixture is cooled from 60-80° C. to 35-50° C. (140-176° F. to 95-122° F.). There is no teaching regarding calcium deposits.
U.S. Pat. No. 5,164,480 describes an electrolytic tall oil soap acidulation process. It discloses that lignates and calcium ions at too high a level may interfere with the process, so pretreatment of the soap is recommended. The pretreatment method is not detailed. There is mention that pretreatment may comprise precipitation, filtration, or precipitation and filtration. In any case, it is “some” (apparently a quite low level of) calcium ions present in the soap and not an insoluble calcium compound (at any level, much less a high level) that is described or of concern here. See, for example, the discussion on the presence of calcium carbonate in the soap given below.
Krumbein (“Efficient Tall Oil Plant Can Benefit Kraft Mills,” Southern Pulp and Paper, August 1984, pp. 36-38) discusses reasons for tall oil soap washing. Although the process described does address removal of lignin and to a limited extent the direct removal of calcium sulfate. However, one skilled in the art would recognize that this process would result in calcium increasing by the fortified brine going to a weak liquor system that returns black liquor to the wash process. Clarification of fortified brine is not discussed.
Vuorikari (“The Occurrence of Foreign Cations in Black Liquor, Soap and Tall Oil Acidulation Systems,” Naval Stores Review, July/August 1992, pp. 4-10) describes a process similar to that in U.S. Pat. No. 4,248,769 (see above) and provides much discussion as to calcium compounds in some systems. It states that soap washing with alkaline tall oil plant brine will not effectively remove calcium when alkaline plant brine is used as the wash medium. There is no mention of clarification of fortified brine, but there is some discussion of where unclarified fortified brine may be sent in the black liquor system.
As mentioned previously, tall oil may be used as a fuel component. WO 2009/131510 describes washing tall oil to remove lignin and calcium soaps prior to using the tall oil in diesel fuel. It teaches that this washing is needed to remove lignin and calcium soaps remaining after acidulation of tall oil soap that then end up in the CTO. These contaminants in the CTO interfere with catalysts used in making diesel.
There is a need for improved methods of making tall oil and associated materials. In particular, in a process for converting black liquor soap to crude tall oil, there is a need for method of removing insoluble calcium and calcium carbonate and lignin from black liquor soap, while improving the yield of CTO from black liquor soap. Removing calcium carbonate will reduce calcium sulfate formation in an acidulation process that uses sulfuric acid. This, in turn, will reduce the amount of calcium sulfate cycling back into a wash medium for washing black liquor soap. Removing insoluble inorganics and lignin will improve the yield of CTO from black liquor by reducing entrained CTO in a spent acid stream coming from an acidulation process, and by reducing inefficiencies caused by deposits on process equipment.